This invention relates to plasma X-ray sources and, more particularly, to sources of soft X-ray or extreme ultraviolet photons, wherein high power production of photons is achieved by electrostatic acceleration of ions toward a plasma discharge region, followed by neutralization of the ions so as to avoid space charge repulsion as the discharge region is approached.
A high power bright source of extreme ultraviolet or soft X-ray photons is required for the process of optical lithography using a scanning ringfield camera or other imaging system. Ringfield lithography is described, for example, in U.S. Pat. No. 5,315,629 issued May 24, 1994 to Jewell et al. The wavelength range of 12.5 to 14.5 nanometers is particularly useful for this purpose, because in this band a relatively highly reflecting mirror based on a silicon and molybdenum multilayer is available. Another nearby wavelength, 11.4 nanometers, is also of interest because of the available silicon and beryllium multilayer mirror.
The xenon band emission between 10 nanometers and 15 nonometers has been proposed for the generation of these lithographic wavelengths via the emission of a plasma created by focusing a pulsed laser onto a xenon cluster expansion. See, for example, U.S. Pat. No. 5,577,092 issued Nov. 19, 1996 to Kubiak et al. The xenon plasma that results must reach a temperature of more than 20 eV (electron volts) in order to contain the highly ionized xenon species that radiate in the desired band. Conversion from laser energy to usable 13.5 nanometer photon energy is less than 1% efficient, with the consequence that a very high power (multiple kilowatt) laser is required. Such lasers have high capital and operating costs. Two other disadvantages of the laser-produced plasma approach are: (a) the collection optical elements must be close to the plasma in order to collect a large solid angle of the emitted radiation, with the consequence that xenon ions from the plasma damage the collection surface by sputtering, and (b) the nozzle that produces the cluster expansion must be within approximately 2 millimeters of the plasma for the process to work. This causes nozzle erosion and leads to material deposition on the collecting optics and subsequent degradation of the extreme ultraviolet reflectivity of the collecting optics.
A more direct method for the generation of 10-15 nanometer xenon band radiation is the magnetic acceleration of xenon ions toward the axis of a pulsed cylindrical discharge known as a Z-pinch discharge. This technique is disclosed, for example, in U.S. Pat. No. 5,504,795, issued Apr. 2, 1996 to McGeoch and in McGeoch, xe2x80x9cRadio Frequency Pre-ionized Xenon Z-pinch Source for Extreme Ultraviolet Lithography,xe2x80x9d Applied Optics, Vol. 37, pages 1651-1658, 1998. The source includes a chamber defining a pinch region having a central axis, an RF electrode disposed around the pinch region for pre-ionizing the gas in the pinch region to form a plasma shell that is symmetrical around the central axis in response to application of RF energy to the RF electrode, and a pinch anode and a pinch cathode disposed at opposite ends of the pinch region. An X-radiating gas is introduced into the chamber at a typical pressure level between 0.1 torr and 10 torr. The pinch anode and the pinch cathode produce a current through the plasma shell in an axial direction and produce an azimuthal magnetic field in the pinch region in response to application of a high energy electrical pulse to the pinch anode and the pinch cathode. The azimuthal magnetic field causes the plasma shell to collapse to the central axis and to generate X-rays.
The Z-pinch source directly converts electrical energy into plasma energy, with relatively high efficiency. Approximately 10% of the delivered electrical energy is radiated in the xenon band. However, because the radiating plasma is several times larger than the one produced by focusing a laser, a smaller solid angle of the radiation can be collected and directed into the lithographic optics which has limited etendue. The net efficiency gain is therefore reduced to a smaller factor, in the range of 2-4 times. An advantage of the smaller collection angle of the Z-pinch source is that the collection optical surface is far enough distant from the pinch region not to be damaged by xenon ions. The smaller collected photon beam angles also allow the insertion, between the plasma and the collection optics, of a foil trap or other device for the removal of contaminants and particulates to protect the collection optics over a long operational life.
A disadvantage of the Z-pinch source arises from the fact that ion acceleration occurs as the result of forces on the electrons in the discharge plasma. The electrons flow in a cylindrical sheet between discharge cathode and anode, and a return current flows through an outer conducting cylinder. Between these cylindrical current sheets, a strong magnetic field provides pressure to accelerate the plasma sheet toward the Z-pinch axis. However, the cost of generating the plasma sheet is that electrons must be extracted from the Z-pinch electrode, and this process is associated with small but inevitable rates of electrode erosion due to sputtering of electrode material by the incident xenon ions from the discharge.
A device known as the fusor, in which ions are accelerated toward the center of a sphere in order to create fusion reactions, has been studied. Such devices are disclosed, for example, in U.S. Pat. No. 3,258,402 issued June, 1966 to Farnsworth, U.S. Pat. No. 3,386,883 issued Jun. 4, 1968 to Farnsworth and U.S. Pat. No. 3,530,497 issued Sep. 22, 1970 to Hirsch et al.
All of the known prior art devices have had one or more drawbacks and disadvantages. Accordingly, there is a need for improved methods and apparatus for generating soft X-ray or extreme ultraviolet photons.
According to a first aspect of the invention, a source of photons comprises a discharge chamber, a plurality of ion beam sources in the discharge chamber, and a neutralizing mechanism. Each of the ion beam sources electrostatically accelerates a beam of ions of a working gas toward a plasma discharge region. The neutralizing mechanism at least partially neutralizes the ion beams before they enter the plasma discharge region. The neutralized beams enter the plasma discharge region and form a hot plasma that radiates photons.
The photons may be in the soft X-ray or extreme ultraviolet wavelength range. In one embodiment, the radiating photons have wavelengths in a range of about 10-15 nanometers.
The ion beam sources may be pulsed or continuous. In one embodiment, the plasma discharge region has a spherical shape and the ion beam sources are distributed around the spherical plasma discharge region. In another embodiment, the plasma discharge region has a cylindrical shape and the ion beam sources are distributed around the cylindrical plasma discharge region.
The plurality of ion beam sources may comprise concentric electrode shells having sets of apertures aligned along axes which pass through the plasma discharge region, a voltage source for applying a voltage between the electrode shells, and a gas source for supplying the working gas to the sets of apertures in the electrode shells. The electrode shells may comprise a cathode shell and an anode shell. The electrode shells may further comprise one or more intermediate shells between the cathode shell and the anode shell. The electrode shells may be configured to produce pseudospark discharges and, more particularly, may be configured to produce tandem pseudospark discharges.
In one embodiment, the neutralizing mechanism comprises resonant charge exchange in each of the ion beams. In another embodiment, the ion beams are neutralized by the introduction of electrons.
The working gas may be selected from the group consisting of xenon, lithium, helium, neon, argon and krypton, but is not limited to these gases. The working gas pressure in the discharge chamber is preferably in a range of about 1-100 millitorr.
According to another aspect of the invention, a photon source comprises a discharge chamber containing a working gas, concentric electrode shells in the discharge chamber, a voltage source for applying a voltage between the electrode shells, and a neutralizing mechanism. The electrode shells have sets of apertures aligned along axes which pass through a plasma discharge region. Beams of ions of the working gas are directed along the axes toward the plasma discharge region. The neutralizing mechanism at least partially neutralizes the ion beams before they enter the plasma discharge region. The neutralized beams enter the plasma discharge region and form a hot plasma that radiates photons.
According to a further aspect of the invention, a system for generating photons is provided. The system comprises a housing defining a discharge chamber, concentric electrode shells located in the discharge chamber, a voltage source for applying a voltage between the electrode shells, a gas source for supplying a working gas to the discharge chamber, a neutralizing mechanism, and a vacuum system for controlling the pressure of the working gas in the discharge chamber. The electrode shells have sets of apertures aligned along axes which pass through a plasma discharge region. Beams of ions of the working gas are directed along the axes toward the plasma discharge region. The neutralizing mechanism at least partially neutralizes the ion beams before they enter the plasma discharge region, wherein the neutralized beams enter the plasma discharge region and form a hot plasma that radiates photons.
The gas source and the vacuum system may be connected to provide circulation of the working gas through the discharge chamber.
The system may further comprise a feedback control system for controlling the rate of flow of the working gas into the discharge chamber in response to a measured spectrum of the radiated photons. The feedback control system may comprise a photon detector for detecting the spectrum of the radiated photons and a flow controller responsive to the measured photon spectrum for controlling the flow of the working gas into the discharge chamber.
The housing may include a structure for passing the radiated photons to a collection region. The structure may comprise a honeycomb screen having a plurality of holes aligned with the direction of propagation of the radiated photons.
According to another aspect of the invention, a method for generating photons is provided. The method comprises the steps of electrostatically accelerating a plurality of beams of ions of a working gas toward a plasma discharge region, and at least partially neutralizing the ion beams before they enter the plasma discharge region, wherein the neutralized beams enter the plasma discharge region and form a hot plasma that radiates photons.